When a wireless communication system for use in a frequency band requiring no license, it is effective to implement a frequency hopping system as means for avoiding or sufficiently suppressing the effect of a narrow-band interference.
FIGS. 6 and 7 show the configurations of the transmitter and the receiver of a wireless communication system when a common frequency hopping system is used. The transmitter obtains a frequency hopping radio modulation signal by inputting to a mixer 4 a signal obtained from an intermediate frequency (IF) band modulator 1 for receiving transmission data and an output of a hopping synthesizer 3 controlled by a signal from a hopping pattern generator 2. An unnecessary wave component is removed from the signal by a band pass filter 5, and a resultant signal is transmitted from a transmitting antenna 6. In the receiver, after a received signal is received by a receiving antenna 7 and amplified by an amplifier 8, an unnecessary wave component is removed by a wide band pass filter 9, the resultant signal and an output signal of a hopping synthesizer 12 controlled by a hopping pattern generator 11 for generating a frequency hopping pattern corresponding to a desired received signal are input to a mixer 10, the signals pass through a band pass filter 13, and a non-hopping fixed IF frequency modulation signal is obtained. The signal is input to an IF band demodulator 14, and the data is demodulated. FIG. 7 shows the configuration for capturing the synchronization of a hopping frequency in the receiver. That is, the output of the band pass filter 13 further passes through a signal detector (envelope detector) 15 and an integrator 16, and is compared with a threshold level and input to a phase controller 18, and the output controls the generated phase of the hopping synthesizer 12.
In this case, to maintain the quality of the transmitted signal, and to obtain a substantial interference suppression effect by a frequency hopping, it is necessary to maintain a predetermined frequency difference between the output of the hopping synthesizer used in a transmitter and the output of the hopping synthesizer used in a receiver, and to maintain small time fluctuation for a phase difference. Therefore, it is necessary that the hopping synthesizer to be used in a transmitter and a receiver is stable in frequency, has low phase noise, and is highly responsive. Especially, in a high-frequency microwave area, various methods for stabilization, lower noise, etc. using a dielectric oscillator or a PLL (phase lock loop) are devised.
However, as the frequency becomes higher (for example, a milliwave band of 30 GHz or more), it is more difficult to realize a frequency-stable and low phase noise hopping synthesizer, and the production cost rises. Furthermore, when a modulation signal to be transmitted is a multicarrier signal which is a narrow-band modulation signal or an OFDM (orthogonal frequency division multiplexing) signal, it is necessary that the hopping synthesizer between a transmitter and a receiver has a synchronous frequency with high precision. Additionally, since the frequency changes at a burst, a responsive countermeasure is required. However, it is very hard to operate in, for example, a high frequency such as a milliwave band a hopping synthesizer capable of synchronize a frequency with high precision, and it is also very difficult to immediately take action against an occurrence of a large frequency offset or synchronously supplement it, thereby requiring an expensive system stabilizing mechanism.